Robert W. Johannsen, "Douglas, Stephen Arnold," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00325.html.
Like many Americans, Douglas hailed the triumphant end of the Mexican War as the beginning of a new era. Events quickly dashed his expectation. The most important consequence of the war was the re-entry of the slavery issue into national politics, raising again the question of slavery's relation to territorial expansion. The introduction of the Wilmot Proviso in the summer of 1846, stipulating that slavery would be barred forever from all lands acquired from Mexico, initiated a bitter sectional debate that increased in intensity until 1850, when the Union itself appeared to be in danger. Douglas rejected both the northern antislavery position that the national government had the power to prohibit slavery in the territories and the southern proslavery argument that the Constitution sanctioned the existence of slavery in the territories. Instead he proposed, as the only fair and just course, to allow the people of the territories to decide the question for themselves without the intervention of the national government. This doctrine of popular sovereignty, Douglas believed, satisfied the yearnings of westerners for self-government and removed the divisive slavery question from national politics. The conflict finally culminated in the passage of the Compromise of 1850, in which Douglas played a leading role. The territories of Utah and New Mexico were organized on the basis of popular sovereignty, and California was admitted to the Union as a free state in accordance with the wishes of its inhabitants.